Capitalisation of English names in the UI should be unified

[Piotrek Koszuliński]

In most places names are formatted like "Language Direction", "Advisory Title", "Split Cell Horizontally". However, some labels are not following this standard - e.g. in the table dialog there's "Border size" and "First column", in the flash dialog there's "Window mode", etc.

This should be unified across the entire editor UI. We also should have this standard written down somewhere (may be a part of code style documentation). Especially that it's not obvious when we write all words capitalised or not. I guess the differentiation is made based on whether this text is a sentence or a label (e.g. error messages are sentences).

[Piotrek Koszuliński]

DUP was reported in #12661.