Text tool - Add leading and detect kerning from font

[Giwayume]

#185

• Fixes line height calculation bug, uses largest ascender and largest descender on the line (if they happen to be two separate characters, one pushing the line up or down further than the other), instead of just finding the one character that has the largest overall height irregardless of the baseline. This was only noticable when you have two fonts where one has a significantly larger descender even at a smaller total line height, such as "Indie Flower" + "Arial".
• Add support to detect and cache kerning pairs from font. Can turn this off.
• Line leading control added, which works as a top offset.
• Updated legacy file conversion code to adjust leading and layer position so the files match as visually close as possible.

[viliusle]

Can we rename kerning to Letter spacing and Leading to line height in GUI ? I believe these terms are easier to understand.

[Giwayume]

@viliusle I'm ok with renaming kerning in the toolbar attributes to letter spacing. Letter spacing is an even kerning adjustment across groups of letters, which kind of works since it applies to the selected letters. But kerning in the layer details needs to stay as kerning, because that is specifically in reference to kerning letter pairs which is the definition of kerning.

Leading and line height are two different things. If it were to be renamed to line height, the value would need to change to a percentage instead of an offset, which I think is a little more difficult to work with.

[viliusle]

Understood. So lets leave as it is. Even if somebody don't know these terms, its very easy to find what it really does.