write support for utables

[ct]

Currently there is no 'write' support for utables. That means that uwiki has no interface to 'set' some metadata and store this in the metadata db.

Adding write support is not completely trivial since we the utable text files are meant to be human readable and shall be preserved and handled in a smart way. That means comments and empty lines shall be preserved. The expansion capabilities of utables shall be used.

For example, given is the utable:

foo bar barf

Then adding foo baz barf shall yield in a 'compressed' table like:

foo bar, baz barf

This is similar to the proposals for the lumiera config loader.

[ct]

switching to sqlite