Initial debian/rules wrote into debian/postit/usr/... directly.
That fails: dh_clean (run between override_dh_auto_build and
dh_install) wipes the package staging dir, and dh_install needs
debian/tmp/ as the canonical staging area to install files
into debian/postit/.
Move the cp-and-convert block out of override_dh_auto_build into
a 'stage_postit' make define, and call it from override_dh_install
right before dh_install moves files into the package tree.
Also: simplify POSTIT_* path variables — drop the duplicated
POSTIT_APP_DIR / POSTIT_LIB_DIR / POSTIT_BIN_DIR / POSTIT_SHARE_DIR
set (they all derived from debian/postit which is now what dh_install
populates).
