Fix build system to that we don't touch the source dir

- src/gollery/ui now holds the source files
- src/gollery/ui-build is a scratch dir used during the build
- src/gollery/static holds the final set of files
Split SCSS in many files
ui: Move handling of browser flipper to browser class
ui: Lazily load map tiles in browser
ui: Only translate nodes that contain text only
ui: Translate title of elements too
New UI theme
add charset