Addon manager: install dependencies (#5376)

Addon Manager: Dependency Installation

The Addon Manager can now attempt to use pip directly to install
required packages as specified in either metadata.txt or
requirements.txt files provided by AddOns. The packages are installed
into FreeCAD.getUserAppDataDir()/AdditionalPythonPackages directory.
Only simplified requirements.txt data is supported: any version
information is stripped out, and only packages are supported (that is,
no pip arguments, included files, etc.). Further, packages are checked
against a list of allowed packages prior to being suggested for
installation. Addon authors should submit a PR to the FreeCAD repo
adding their requirements to the allowed list, for packages that are not
already on the list (this is a malware-prevention mechanism).

src/Mod/AddonManager/AddonManagerOptions.ui

@@ -6,7 +6,7 @@
    <rect>
     <x>0</x>
     <y>0</y>
-    <width>388</width>
+    <width>757</width>
     <height>621</height>
    </rect>
   </property>
@@ -232,6 +232,42 @@ of the line after a space (e.g. https://github.com/FreeCAD/FreeCAD master).</str
      </layout>
     </widget>
    </item>
+   <item>
+    <layout class="QHBoxLayout" name="horizontalLayout_2">
+     <item>
+      <widget class="QLabel" name="fclabel">
+       <property name="text">
+        <string>Python executable (optional):</string>
+       </property>
+      </widget>
+     </item>
+     <item>
+      <widget class="Gui::PrefFileChooser" name="gui::preffilechooser" native="true">
+       <property name="sizePolicy">
+        <sizepolicy hsizetype="Expanding" vsizetype="Preferred">
+         <horstretch>0</horstretch>
+         <verstretch>0</verstretch>
+        </sizepolicy>
+       </property>
+       <property name="minimumSize">
+        <size>
+         <width>300</width>
+         <height>0</height>
+        </size>
+       </property>
+       <property name="toolTip">
+        <string>The path to the Python executable for package installation with pip. Autodetected if needed and not specified.</string>
+       </property>
+       <property name="prefEntry" stdset="0">
+        <cstring>PythonExecutableForPip</cstring>
+       </property>
+       <property name="prefPath" stdset="0">
+        <cstring>Addons</cstring>
+       </property>
+      </widget>
+     </item>
+    </layout>
+   </item>
    <item>
     <spacer name="verticalSpacer">
      <property name="orientation">
@@ -273,6 +309,11 @@ of the line after a space (e.g. https://github.com/FreeCAD/FreeCAD master).</str
    <extends>QLineEdit</extends>
    <header>Gui/PrefWidgets.h</header>
   </customwidget>
+  <customwidget>
+   <class>Gui::PrefFileChooser</class>
+   <extends>QWidget</extends>
+   <header>Gui/PrefWidgets.h</header>
+  </customwidget>
  </customwidgets>
  <resources/>
  <connections/>