Merge branch 'sev/small_fixes' into 'master'

Small Fixes

I've had a look at some of the recent changes, great to see so much progress.
This branch adds some very minor fixes for things I noticed:

**Typo**

Fixed typo in the menu (the string is not yet part of the translation files)

**Screenshot viewer controls**

In analogy to the maplist and serverlist, I've added the possibility to open screenshots with LMB and space.

**Arc notification**

The arc kill notification still uses the hlac image, I've updated it

**Slider direction**

Currently, mousewheeldown and the PgDn key move the slider up, while
mousewheelup and the PgUp key move the slider down.
This doesn't make sense, so I've inverted the controls.

**Scrollbar thumb minimum height**

The scrollbar image is subdivided into 3 segments with a ratio of 1:2:1.
While the middle segment is intended to be stretched, the outer segments are not.
The current minimum height for the scrollbar thumb is 1*width, causing the thumb to be cut off, as seen in the advanced settings dialog.
I've changed the minimum height to fully include all "unstretchable" segments.

![minfactor](https://gitlab.com/uploads/xonotic/xonotic-data.pk3dir/402cf3bdc2/minfactor.png)

See merge request !52