Merge branch 'sev/hud_weapon_panel' into 'master'

Merge branch 'sev/hud_weapon_panel' into 'master'

A few HUD weapon panel improvements

Currently, the HUD weapon panel distributes the weapon icons very akwardly. The scaling is especially off when arranged vertically, sometimes leaving a gap on either side. So I've made to following changes:

- Reimplemented the distribution of the icons to better use the available space, and to be more predictable.
- Added a new cvar *hud_panel_weapons_label_scale*, to change the label size.
- Lastly, I've tightened up the HUD editor selection border for all panels, to more closely resemble the selection border in editors like GIMP or Inkscape.

![hud_weapons_v](https://gitlab.com/uploads/xonotic/xonotic-data.pk3dir/0c951a653d/hud_weapons_v.png)

![hud_weapons_h](https://gitlab.com/uploads/xonotic/xonotic-data.pk3dir/e494180790/hud_weapons_h.png)

See merge request !85