All things that can affect attribute from current fit/ships is included in modifiedAttributeDict.

The previous method was to only add those that change a value. This adds things can can affect it, and then pass a new `used` flag to the dict to show if it's used or not.
Add GUI support for changing affected skills
Added default value for `used` parameter
Add icons to skill context menu
Ensure that we load all skills for a character and some minor tweaks.
Merge pull request #110 from blitzmann/issue-101

Add "Change affecting skill" context menu, resolves #101