Tidy, polish or completely update style and preset icons

[skyper]

I have started to create some icons the last month and found many room for updates in clean-ups within the svg source code. See e.g. my comment 13 on #21398 but also #20530 and #21440.

• many icons are still 90 dpi but for screen display 96 dpi is better
• file size can be drastically lowered as
  • we only need plain svg icons
  • icons contain unused elements
  • default values/settings can be removed
• some icons contain strange decimal numbers with a lot of digits behind the separator. This does not make sense with icons of 16x16 px.

Sadly, I did not find a script which helps in cleaning up the soucecode of the icons, yet.

Regarding the actual look, icons could be more harmonized in style and colors, at least compared to similar icons, like barrier icons.

Let's keep track in this ticket and maybe check folder per folder under trunk/resources/images/presets