CSV export for plots

[doluk]

Additions

1. Adds csv export QToolButton to US_Plot with a new icon
2. Adds csv export function to US_GuiUtil similar to save_png

Changes

1. Fixes a minor visual inconsistency in US_Plot caused by the different sizes of buttons and button labels by explicitly setting the size of the buttons.
2. Fixes an edge case bug in the us_plot3d csv export when plotting a model (plus some extra conditions) resulting in an empty csv file.

[demeler]

One question for Lukas: How does this new feature function for plot windows where there are multiple plot elements in the plot? Or multiple CSV files in different formats that should be exported to capture all of the information in the plot? For example, in a van Holde - Weischet distribution plot we have the extrapolation points and the equations for the linear fits, both elements follow a different structure and couldn't really be saved as a single CSV file.

[demeler]

Plot examples from exported data look identical to the original rendition in UltraScan.

[demeler]

Weird, when I pulled #128 again it worked just fine - I can confirm that the csv icon now shows up in Plot Norm program, and it works. Thanks Lukas!