Added Context::extensionStrings().

Returns list of _all_ extensions reported by the driver, even those that
are not supported in Magnum and also those that are marked as disabled
due to driver bugs. Usable mostly when doing fresh port to new platform
to discover possible new features.

The actual implementation was part of Context constructor (and now this
function is called instead). It is rather ugly because we need to take
care of both GL 2.1 and GL 3.0 (and also ES2 and ES3).
Platform: doc++.
Platform: be less greedy in bit flags.
Platform: consistent naming.
Platform: initial support for touch/mouse events in AndroidApplication.

The API now mimics desktop (mouse), might be changed in the future to
properly support multitouch etc. features.
GLSL ES 3.0 requires `es` in version directive.