Fix select new file in Tool Spec Editor

[PiispaH]

If the spec editor of a tool that has no specification is opened, the text editor widget is completely empty and the header is changed into "No file selected". When a main program file is created, it is automatically selected and the text editor is ready for use. +fixed a bug that sometimes when creating a new main program file, the file would be created, but it would not appear in the spec editor.

Re spine-tools/Spine-Toolbox#2099

Checklist before merging

• [x] Documentation (also in Toolbox repo) is up-to-date
• [x] Release notes in Toolbox repo have been updated
• [x] Unit tests have been added/updated accordingly
• [x] Code has been formatted by black
• [x] Unit tests pass

[codecov-commenter]

Codecov Report

Patch coverage: 60.00% and project coverage change: +0.03 :tada:

Comparison is base (4841155) 54.70% compared to head (a895ff2) 54.74%.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #131 +/- ## ========================================== + Coverage 54.70% 54.74% +0.03% ========================================== Files 192 178 -14 Lines 17328 17169 -159 Branches 2841 2822 -19 ========================================== - Hits 9480 9399 -81 + Misses 7358 7282 -76 + Partials 490 488 -2 ``` | [Impacted Files](https://app.codecov.io/gh/spine-tools/spine-items/pull/131?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=spine-tools) | Coverage Δ | | |---|---|---| | [...s/tool/widgets/tool\_specification\_editor\_window.py](https://app.codecov.io/gh/spine-tools/spine-items/pull/131?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=spine-tools#diff-c3BpbmVfaXRlbXMvdG9vbC93aWRnZXRzL3Rvb2xfc3BlY2lmaWNhdGlvbl9lZGl0b3Jfd2luZG93LnB5) | `44.09% <60.00%> (+2.36%)` | :arrow_up: | ... and [15 files with indirect coverage changes](https://app.codecov.io/gh/spine-tools/spine-items/pull/131/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=spine-tools)

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