Move to meson build system

Related to #132, just a rebase on top of current master.

Currently does not work any more on my computer, because I had to add merging translations to appdata file and I'm getting:

[krnowak@localhost mbuild]$ ninja
[1/3] Generating yelp.appdata.xml_merge with a custom command.
FAILED: data/yelp.appdata.xml 
/usr/bin/python3 /home/krnowak/usr/bin/meson --internal msgfmthelper ../data/yelp.appdata.xml.in data/yelp.appdata.xml xml /home/krnowak/projects/yelp/po
msgfmt: cannot locate ITS rules for ../data/yelp.appdata.xml.in
[2/3] Generating symbol file 'libyelp/yelp@sha/libyelp/libyelp.so.0.0.0.symbols'.
ninja: build stopped: subcommand failed.

I have appdata.its and appdata.loc installed in /usr/share/gettext/its/, so I have no clue what's wrong.