build: Rename PROGSHAREDIR to JAMI_DATADIR.

This name matches a common naming convention in build systems.

* compat/msvc/config.h (PROGSHAREDIR): Rename to...
(JAMI_DATADIR) ... this.
* globals.mk (-DJAMI_DATADIR): Likewise.
* meson.build (conf): Likewise.
* src/account.cpp: Likewise.
* src/manager.cpp (Manager::playRingtone): Likewise.

Change-Id: I123251f3e420948bc7359ce0a3ac54bddd12977a