Add basic theme support and switch config to hex colors.

[simotek]

• This moves the text and graph colors along with fonts into a separate file / module and then loads them into a current_theme variable so that its easy to add and swap between colors and fonts, we can also build up and add more style options later.
• It also swaps the widget api and therefore theme from using cairo format colors to using conky and or html hex format colors and does the conversions internally.

I suspect that it will be best if I rebase this one after the other PR's are accepted.

I also made the following as a second different example that can be used for testing. But I figured there is no point in pushing it yet because it doesn't really have a matching conky config yet. https://github.com/simotek/conky-bubbles/blob/dev/src/themes/dimensions.lua

[simotek]

I marked this one as a draft because I'll need to rebase it at some point once some of the other changes are in.