UI Tweaks for all mobile platforms.

* PX4: Deal properly with optional components

* RC map UI: Offer better defaults and user guidance

all calls to qsTr are done, if I didn't forget anything,
on the PX4 QML files.
Signed-off-by: Tomaz Canabrava <tomaz.canabrava@intel.com>

Add scroll height indicators to Flickable

Also adjusting text layout while at it.

Everything is now based off of default font pixel size

Use default font wherever possible. Where that doesn’t work, use
ScreenTools::dpiAdjustPointSize.

Moved update canvas signal to perspective load instead of hooking it to the paint event.
Added an "S" as the unit for the number of cells fact (as in 4S).