Problems with the explicit Extract scale button

Some issues with commit 7cb84bac (explicit button for scale extraction from markers):

Scenario A:

  1. Start application, open existing project, File/Import photos...

    • Browse button is enabled,
    • Extract button is disabled, text is "Extract scale from scale markers",
    • Import button is disabled,

    all as expected.

  2. Browse for a folder with a.png (the photo contains a scale marker).

    • Browse button stays enabled,
    • Extract button gets enabled, text stays "Extract scale from scale markers",
    • Import button gets enabled,

    all as expected.

  3. Click "Extract scale from scale markers".

    • Browse button gets disabled (expected),
    • Extract button stays enabled, text changes into "Cancel scale extraction" (expected),
    • Import button stays enabled (unexpected), text changes into countdown (expected).

  4. Wait for the extraction to finish.

    • Browse button stays disabled (unexpected),
    • Extract button stays enabled, text stays "Cancel scale extraction" (unexpected),
    • Import button stays enabled (would actually expect to be disabled from step 3) until now), text changes into "Import" (expected).

  5. Click the "Cancel scale extraction" button. Unexpectedly, it seems to start the extraction, like a repeat of step 3). After the scale extraction finishes, the Browse button is still disabled, and this button text is still "Cancel scale extraction".

The only way to re-enable the Browse button seems to be to click the "Cancel scale extraction" button while the extraction is actually running.

Scenario B:

Do steps 1) (including a new start of the application) to 4) as in scenario A, then click "Import". Then File/Import photos... again -- unexpectedly, when the Import photos window opens, the Extract button is disabled, but its text already says "Cancel scale extraction".

The Extract button in a freshly re-opened Import photos window will incorrectly say "Cancel scale extraction" also if the Import button during the previous import was clicked while the extraction was still running (and the Import button was displaying the countdown and probably should have been disabled).