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)
Loading the icon for the button from resources ceased to work (maybe when
switching to `PlasmaComponents3.ToolButton`). This change works around the
problem and the old icon no longer needs to be bundled.