use icons for pos/rot/scale animation key buttons

• implements the icons for the animation key buttons.
• renames key_loc_button to key_pos_button

However I'm a bit unhappy about the current result. If you read this PR you'll see I started by matching the current buttons, which is why I made enabled/disabled pairs of icons in the beginning. This does not fit very well with the current theme though, which is flat with a blue overlay for toggled icons.

• with both types: Confusing, looks like the scale key button is disabled and not just toggled off.
• with enabled type only: Also confusing, the scale key doesn't look toggled off. It gets really non-obvious I think with everything toggled off.

It seems to me the problem is that the button style does not look togglable and the overlay is inadequate for that.

A quick workaround is to set/unset flat but that's a bit of a hack:

Thoughts?