Main Window Layout bugs

[davidrg]

The main window has a number of minor UI layout issues including:

• Current conditions labels don't have enough vertical space (the g in "light breeze" is being cut off)
• The image panel isn't resized properly on show if the application is started with hit hidden due to there being no active image sources

Switching from a Davis to an inactive FineOffset station results in the UI not resizing properly leaving lots of empty space too. The resize doesn't happen until first current conditions update which never happens - need to find a way of doing this sooner.

Really these are side-effects of the fixed layout the main window currently uses. The code for it is fragile and inflexible. The proper solution is probably to remove the fixed layout entirely as its likely a major source of problems for #12.

[davidrg]

The layout issues are worse on linux where a few elements aren't aligned properly.

[davidrg]

Qt 6 now obeys the system font settings on windows which makes this worse too.