Fix indentation.
Update .clang-format.
Initial work on supporting floating text.

- In order to support more flexible layouts, the text item doesn't store
an absolute position like it did in v1.7. Instead, it's now tied to a
position in a system.
- Added a TextItem class which just stores the position and the contents
of the text. Eventually this will probably store a font setting as well.
- Bumped up the file format version.

Bug: #12
Copy text items when merging scores.

Bug: #12
Add basic rendering of text items in the score.

Bug: #12
Import floating text from v1.7 files.

- The absolute positions from v1.7 are converted into a system +
position combination so that they will work with flexible layouts.
- This required adding back some code for dealing with layouts in v1.7
files that I removed a while ago.

Bug: #12