Fix incorrect tuning when using LNB LO.

Fixes the wrong behaviour introduced by commit 58b527320cabe19c15d84373e750747681dd29ce
and later.
Update tooltips.
Fix hangs and segfaults when the FFT is moved around and zoomed

Moving the FFT plot off the screen caused a hang or a segfault. Making
the window too large caused segfaults because MAX_SCREENSIZE and
HORZ_DIVS_MAX were not taken into account. The zoom could also become
negative or get stuck on max.
Limit the max zoom-out to fix a crash

If the fft plot is zoomed out too far then there is an integer
overflow when computing m_pTranslateTbl, which results in memory
corruption when writing to outBuf.
Place limits on the fft plot dB range zoom

The fft plot dB range would get stuck if zoomed in or out too far. Put
limits on the range to prevent this from happening.
Merge pull request #65 from weinholt/master

Various fixes for a UI problems.
Restore correct receiver frequency.

Wait with updating the GUI until last to prevent applying frequency
offset multiple times.
Merge branch 'master' into fft_filter