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### Problem description
This PR aims to address #1645 that caused the built in file provider's
change monitor to trigger the notification popup dialog multiple times
in a row after multiple external file changes.
### Implementation description
I added an additional boolean field
`m_changeEventAcknowledgementPending` that tracks whether there are any
pending or unacknowledged change notification dialogs to prevent further
dialogs from being opened. The flag is only reset to its initial value
once the user has acknowledged the first `PopupQuestion` dialog.
Since the file is reloaded only after the user clicks 'Yes', it is
unnecessary to ensure that only the latest popup is acknowledged.
### Problem description
Ref #1210
### Implementation description
Call
[`SetFileTime()`](https://learn.microsoft.com/en-us/windows/win32/api/fileapi/nf-fileapi-setfiletime)
everytime `FileProvider::save()` is called.
### Additional things
I moved the call to `File::close()` from `FileProvider::open()` to
`FileProvider::close()` because `SetFileTime()` requires a file handler
as input, so I need `File::m_file` to be valid.
In order to do this I add to make some other additions :
- Add a warning popup (TODO, maybe add some icons to differentiate
error/warning popups in a future PR ?)
- create showError() and showWarning() functions, as helpers to show a
message both to the logs and as a popup
This PR does two correlated things:
- Show a generic error message (Failed to open provider) rather than a
file-specific message (Failed to open file) when a provider fails to
open
- Set the error to something more specific when opening a file fails
