Inconsistent handling of colour palettes in VISAT colour manipulation window

The VISAT colour manipulation window is currently inconsistently handling the sample values assigned to a slider in a colour palette, especially if the statistics switch from inexact to exact ones. Reproduce:
* Open band image
* Note current slider settings
* Press 95% button
* Note different slider settings now
* Compute exact statistics (see [BEAM-892])
* Note different slider settings now
* Press 95% button
* Note different slider settings now

Even more important is the suspicious behaviour if colour palettes are exported and the imported again. Reproduce:
* Open band image
* Note current slider settings
* Export colour palette 'test'
* Compute exact statistics (see [BEAM-892])
* Import colour palette 'test'
* Note different slider settings now