[partition] Warn if the system won't support encryption in a useful way

This is the infrastructure bit; if someone can come up with a way
of **meaningfully** detecting support, the detection function can
be given a better implementation.

FIXES #1725

src/modules/partition/gui/EncryptWidget.cpp

@@ -13,9 +13,26 @@
 
 #include "ui_EncryptWidget.h"
 
+#include "Branding.h"
 #include "utils/CalamaresUtilsGui.h"
 #include "utils/Retranslator.h"
 
+/** @brief Does this system support whole-disk encryption?
+ *
+ * Returns @c true if the system is likely to support encryption
+ * with sufficient performance to be usable. A machine that can't
+ * doe hardware-assisted AES is **probably** too slow, so we could
+ * warn the user that ticking the "encrypt system" box is a bad
+ * idea.
+ *
+ * Since we don't have an oracle that can answer that question,
+ * just pretend every system can do it.
+ */
+static inline bool systemSupportsEncryptionAcceptably()
+{
+    return true;
+}
+
 EncryptWidget::EncryptWidget( QWidget* parent )
     : QWidget( parent )
     , m_ui( new Ui::EncryptWidget )
@@ -27,6 +44,18 @@ EncryptWidget::EncryptWidget( QWidget* parent )
     m_ui->m_passphraseLineEdit->hide();
     m_ui->m_confirmLineEdit->hide();
     m_ui->m_iconLabel->hide();
+    // TODO: this deserves better rendering, an icon or something, but that will
+    //       depend on having a non-bogus implementation of systemSupportsEncryptionAcceptably
+    if ( systemSupportsEncryptionAcceptably() )
+    {
+        m_ui->m_encryptionUnsupportedLabel->hide();
+    }
+    else
+    {
+        // This is really ugly, but the character is unicode "unlocked"
+        m_ui->m_encryptionUnsupportedLabel->setText( QStringLiteral("🔓") );
+        m_ui->m_encryptionUnsupportedLabel->show();
+    }
 
     connect( m_ui->m_encryptCheckBox, &QCheckBox::stateChanged, this, &EncryptWidget::onCheckBoxStateChanged );
     connect( m_ui->m_passphraseLineEdit, &QLineEdit::textEdited, this, &EncryptWidget::onPassphraseEdited );

src/modules/partition/gui/EncryptWidget.ui

@@ -37,6 +37,19 @@ SPDX-License-Identifier: GPL-3.0-or-later
      </property>
     </widget>
    </item>
+   <item>
+    <widget class="QLabel" name="m_encryptionUnsupportedLabel">
+     <property name="toolTip">
+      <string>Your system does not seem to support encryption well enough to encrypt the entire system. You may enable encryption, but performance may suffer.</string>
+     </property>
+     <property name="text">
+      <string notr="true">🔓</string>
+     </property>
+     <property name="alignment">
+      <set>Qt::AlignCenter</set>
+     </property>
+    </widget>
+   </item>
    <item>
     <widget class="QLineEdit" name="m_passphraseLineEdit">
      <property name="echoMode">
@@ -57,6 +70,19 @@ SPDX-License-Identifier: GPL-3.0-or-later
      </property>
     </widget>
    </item>
+   <item>
+    <spacer name="horizontalSpacer">
+     <property name="orientation">
+      <enum>Qt::Horizontal</enum>
+     </property>
+     <property name="sizeHint" stdset="0">
+      <size>
+       <width>40</width>
+       <height>20</height>
+      </size>
+     </property>
+    </spacer>
+   </item>
    <item>
     <widget class="QLabel" name="m_iconLabel">
      <property name="text">