* Make the Fork Awesome image provider URL a property and change its value
if the Plasmoid's theme changes to cause Qt Quick to redraw images
* Turn caching of Image objects off so Qt Quick actually fetches the images
again from the image provider
* Change the URL only back after returning to the event loop; otherwise Qt
Quick apparently does not react to the property change
* See https://github.com/Martchus/syncthingtray/issues/126
This should avoid:
```
file:///usr/share/plasma/plasmoids/martchus.syncthingplasmoid-git/contents/ui/main.qml:13:34: QML FullRepresentation: Binding loop detected for property "implicitWidth"
…
qml: PlasmaExtras.ScrollArea is deprecated. Use PlasmaComponents3.ScrollView instead.
…
file:///usr/share/plasma/plasmoids/martchus.syncthingplasmoid-git/contents/ui/TabButton.qml:27:13: Unable to assign [undefined] to QFont
…
file:///usr/share/plasma/plasmoids/martchus.syncthingplasmoid-git/contents/ui/ToolButton.qml:23:13: Unable to assign [undefined] to QFont
```
* Use header (with integrated buttons when shown as part of the system tray
Plasmoid)
* Use less space so it fits into the system tray plasmoid (at least on a
full HD screen with 96 dpi)
* Show action for internal errors only if there are internal errors (like
in the Qt Widgets based GUI)
* Port away from deprecated tab bar (which is not used in official
Plasmoids anymore as well)
* Simplify code for ensuring the minimum size as configured (still does
not work within the system tray Plasmoid)
Martchus