Feature form redesign

[tomasMizera]

These things are handled:

• Text form editor
• Text multiline form editor
• Calendar form editor
• Value relation form editor
• Value map form editor
• Photo form editor
• Gallery form editor
• Relation form editor
• Switch form editor
• Slider form editor
• Number form editor
• Remove old code
• Form itself in new design (partially done - in gallery)
• Fix gallery
• Preview panel - minimal though and needs a bit of tweaking
• Remember last entered values

Not handled:

• Relation reference form editor
• Text widget
• HTML widget
• Spacer widget

Sneak peek

• As a side effect, fixes Do not allow edit mode for read-only layers in the feature form #2870
• Added a hack to choose the relation editor type. Enter nogallery into relation editor title and it will use word tags, not gallery; fixes 1-N relation not showing display text #2121
• Note: Font colors for editors in readonly form state are too bright <<--- needs adjustment