debian/control: bumping build-depends for debhelper

Since moving to debhelper as buildsystem we have to increase the
build depend on debhelper to version >= 7.0.50~ as we use the debhelper
override function. This is need at least debhelper >= 7.0.50.

This fixes Lintian warning
lintian: fixing debhelper-overrides-need-versioned-build-depends (>= 7.0.50~)
debian/gbp.conf: adopting changes for Debian experimental branch
Imported Upstream version 28.0~b1
Merge tag 'upstream/28.0_b1' into experimental

Upstream version 28.0~b1
rebuild patch queue from patch-queue branch

obsolete patches (fixed upstream):
debian-hacks/Do-build-time-detection-of-2-bytes-wchar_t-and-char1.patch
debian-hacks/Fix-build-failure-for-header.py-and-typelib.py.patch
debian/control: add libpulse-dev build dependency

New Thunderbird version needs now a build dependency on libpulse-dev.
debian/control: bumping some version of build dependencies

The packages nspr4-dev, sqlite-dev and libvpx-dev needs at least to be
present in a higher versions.
debian/icedove-dev.install: adopt upstream changes

Mozilla has changed the old directory order and release of files in
there. Remove no longer shipped files and changed source of remaining
files.
icedove.lintian-overrides: adding libtheora

Mozilla needs a own currently pre-released version of libtheora.
remove file temporary
debian/rules: adding removing for temporary files

The build inside iceowl-extension and calendar-google-provider contains
sometime a temporary stamp file ".mkdir.done". Lets remove this files.