Importer: random usability improvements

[jkiviluo]

• [x] When user writes text in the source ref field, then Toolbox should guess that the source type is a constant
• [x] When the user puts an integer in the source ref field, then Toolbox should guest that the source type is a column (it could also be row, but it's more likely to be column and it's also easier to use keyboard to get 'row' in the source type since it's the only one that starts with 'r' - with column there are three options).
• [x] Updated: Toolbox could keep the content of Source ref even when source type is changed. E.g. changing from column to constant could still hold the integer. If it's incompatible, it can be painted red (and if the type is None, then it's red and not used). What happens is that you accidentally put the wrong type and you lost your spelling.
• [ ] Default width of 'source type' could be wide enough to hold the longest type: 'column header'
• [ ] Default width of Source ref could be much wider (sometimes those relationship class names are pretty long). Couldn't it be rest of the mapping specification tab width?
• [ ] The widths of different columns in the source data tab could be remembered for each source data table. I guess changing the structure (e.g. column to rows) can destroy the nice view, but so be it. Most of the time it remains the same.

Could have also been added in #367, but maybe better to make this as it's own.

[jkiviluo]

I ticked the third box - it's not done, but the undo remembers the old name.

[jkiviluo]

Let's finish this for v0.7 - I think the remaining things are small UI changes.

[jkiviluo]

Moving to 0.8 after all in order to avoid merge issues.