[mobileshell]: allow scaling status bar

This change allows scaling of the statusbar (the top panel).

As we have to accommodate for a wide variety of devices with different physical pixel sizes, we need a way to scale the top panel / status bar. This change introduces a config value (in plasmamobilerc, such as other shell settings) that controls this size.

For example (plasmamobilerc):

[General] statusBarScaleFactor=1.5

MR also contains config UI in kcm mobileshell.

\Updated in latest iteration:

• rounded text pixel sizes
• rounded status bar size to even values to aid in alignment and vertical spacing
• lockscreen also uses scaled value
• removed smallest value in kcm, this produces a panel so small that alignment can get messed up: don't provide an option that makes us look bad, and the panel is quite small, anyway

Normal (1.0):

Small (0.75) (removed in latest iteration):

Large (1.5):

Very large (2.0):