Add support for restoring scroll position of file lists

[guanglinn]

This PR mainly implemented the following features for file browser:

1. Highlight recently opened files or folders;
2. Can restore the scroll position for the upper level list.

The screen recording as follows:

restore-position-and-highlight.mp4

[harshad1]

We already have logic to flash a view and this is used in other places.

markor/app/src/main/java/net/gsantner/opoc/util/GsContextUtils.java

Line 2853 in 60d1f43

public static void blinkView(final View view) {

See how it is used here:

https://github.com/gsantner/markor/blob/master/app/src/main/java/net/gsantner/opoc/frontend/filebrowser/GsFileBrowserListAdapter.java#L550

We already have a function to show and blink a file in the folder view. Just need to call it at the appropriate time.

I have added an option to show the currently open file in the file explorer in this PR:

https://github.com/gsantner/markor/pull/2176/files#diff-a5f18ed8d9a7afd47e78969d79f34ea509f949c4f383113680d828316380943dR607-R610

An option has been added to the file menu to show the file in the file explorer. This works even if the file was launched from search or a shortcut.

This PR may not be necessary

[guanglinn]

@harshad1 , Does your PR have the feature to restore scroll position of file list? If not, I can only keep this feature in this PR.

[guanglinn]

In addition, I hope the feature in your PR can highlight recently opened folders, rather than only documents.

[harshad1]

No. All I have is the ability to jump from an open file to it's location in the folder.

I also like how the highlighting to show the file / folder looks. There are some good changes in this PR. I will think about this more.

[guanglinn]

I'm still not sure which code should be kept in my PR. Perhaps, I think I can decide it after your PR #2176 is merged.

[gsantner]

PR has conflicts. might be easier to take version from msater and just make the few line changes again

[guanglinn]

@gsantner . Conflicts has been resolved.

[gsantner]

@guanglinn @harshad1

Many merge requests - regarding blinking and showing position in the list: Let's bring it all together on master now, and further improve/debloat it afterwards.

When dark theme is used, I find the white blink color bad.

[guanglinn]

When dark theme is used, I find the white blink color bad.

I will improve it.