Added support annotation library dependency.

The specific reason for this is that it provides @Null and @NotNull
annotations which should increase the safety of our code. Many of the
bugs which get filed are due to NullPointerExceptions, which could be
avoided by tooling using these annotations. The goal is to statically
catch this specific class of errors in as many situations as possible,
rather than waiting for them to occur at runtime.
Clean up build scripts to make it harder to fail.

 * Don't apply android plugin in root project

This results in the root project being treated like and Android project.
That is, gradle will expect an AndroidManifest, a targetSdk property, and
all sorts of stuff that is not relevant to the root project.

Perhaps more importantly, this breaks integration with Android Studio,
which is the tool that many potential contributors will be using.

Finally, it also allows runing gradle tasks in the root project, rather
than having to cd into the F-Droid directory, which is a minor nicety.

The reason it was there in the first place was to make it so that we could
find the location of the Android SDK using the same mechanism that the
plugin used. To deal with this, this commit adapts the SDK finding code
from the gradle plugin.

 * Make gradle error out when missing depenencies.

The support v4 library requires some obsolte SDKs that are likely
not installed. It caused non-intuitive errors to come up for me,
so I've made gradle tell the user when this occurs.

 * Documented the main build.gradle file

This is primarily to explain the hacks we use in order to build the
Android support libraries.
Translation updates
Merge branch 'gradle-improvements' of https://gitlab.com/pserwylo/fdroidclient
Remove all trailing whitespaces in source files
Run fix-ellipsis
Also generate txt lint report for script usage
Fix lone tabbed line
Adapt remove-unused-trans to gradle, yet to be tested
Fix a StringFormatInvalid error
Fix default locale lint warning
Add changelog for 0.78
Updated changelog with "swap" user interface stuff.
Remove more trailing whitespaces
Don't end changelog entries with periods
Bump to 0.78