Add buffer size API and UI for trends

[cpascual]

• Add bufferSize() and setBufferSize() to TaurusTrendSet
• Add BufferSizeTool (and use it in TaurusTrend by default`)
• Add -b / --buffer option to tpg trend CLI

[guifrecuni]

Thanks Carlos the spinbox option looks good. There seems to be some issues while changing sizes (which may go to downsizing while editing)

[cpascual]

For the problem of accidentally losing info due to buffer clearing when downsizing, here are possible mitigators:

• show a warning dialog requesting confirmation before downsizing
• do not use valueChanged but editingFinished to trigger a change
• do not completely clear buffers, keep the last n data (being n the new size)

Also, we found a serious issue: buffer clearing resets curve properties (color, symbols, etc are lost)

[cpascual]

Regarding this problem, I addressed it by allowing downsizing without total wipe and changing from a spinbox to a dialog for entering the new value