[mapplauncherd] Remove applauncherd and refactor boosters into independent processes

Having a single applauncherd process with boosters as plugins is a
flawed idea: most notably, it's not possible to have two boosters with
conflicting libraries, like Qt4 and Qt5.

A much cleaner solution is to turn libapplauncherd into a support
library for boosters, and have each booster run as an independent
process.
[mapplauncherd] Put booster sockets under XDG_RUNTIME_DIR

This is a compatibility-breaking change with boosters. The socketId
field is now expected to be a socket name, not a path. Sockets are
placed under ${XDG_RUNTIME_DIR}/mapplauncherd/, defaulting to /tmp/.

Invoker currently expects sockets in that path and named 'booster-X'
where X is the single character booster type. That is expected to
change when types are refactored to use full strings.
[mapplauncherd] Change booster type to strings

Single-character booster types could potentially conflict when
using out-of-tree boosters, and quickly become confusing.

Old types are mapped for compatibility.
[mapplauncherd] Remove Booster::boosterTemporaryProcessName
[mapplauncherd] Install mapplauncherd.target for systemd startup
Rename ebooster to booster-generic