Enable customization of text alignment in exhibits

[genedan]

The default style is to have all values centered vertically. Horizontally, text is center-aligned and numbers are right-aligned. This is just my personal preference, but others may want something else and we should add features to let that happen:

I propose we can let this happen via context menu when the user right-clicks a column in the exhibit. There are other ways to do this, such as presenting the option via right-clicking the output tree or providing a button on the right side of the dialog box that pops open a design window.