GUI: Reworked modern theme
After patch 1743196 I've begun reworking the modern theme so it shows correct separations between the widgets without the need of extra spaces in the strings. The result is attached.
I ended substituting numeric constants with variables to configure labels width and separations between widgets. It makes the widgets look better aligned and the different tabs look more homogeneous. The two attached screenshots show the difference in the audio tab.
This work is unfinished, just to know your opinion. I've just modified the modern theme (not the classic) and I've just worked with 2x mode. If you like this I can also rework the classic theme. I think the parameter variables would allow the modern theme to reuse bigger parts of the classic.
Ticket imported from: #1743201. Ticket imported from: patches/809.